1. Field of the Invention
The invention relates to a graphics display unit.
2. Description of the Related Art
A conventional graphics display unit is arranged by, for instance, a frame buffer 1a for storing display image data in unit of a pixel (will be referred to as "pixel data" hereinafter), an image drawing unit 2a, a display unit 3a, and a D/A (digital-to-analog) converter 4. The respective pixel data which have been written and then saved into the frame buffer 1a by the image drawing unit 2a are sequentially read by the display unit 3a every display time period of a display unit. Then, an image is displayed on the display unit by processing the pixel data via the D/A converter 4. As a result, in such a case that the display resolution of this display unit is high and also a total number of display colors is large, the memory bandwidth required to read/display the pixel data is considerably increased. In such a graphics display unit that a single port memory is employed as the frame buffer, since this single port is commonly used in the pixel data display/read operation and the image drawing access operation, if the memory bandwidth required to display/read the pixel data is increased, then the image display access operation is restricted. As a consequence, the image drawing performance of this graphics display unit is considerably lowered. One typical example of the above-described conventional display apparatus is described in "MediaGX Targets Low-Cost PCs" written by Linley Gwennap, MICRODESIGN RESOURCES, Volume 11, No. 3, Mar. 10, 1997.
To solve the above-explained problem of the single port memory, there is a method for using a dual port memory in which both a pixel data display/read operation and an image drawing access operation can be performed by two independent ports. However, there is a difficulty such that the dual port memory requires very high cost, as compared with the single port memory.
The present invention has been made to solve the above-described drawbacks, and therefore, has an object to provide such a graphics display unit capable of increasing image drawing performance, while using a single port memory. That is, a compression data storage region for saving compression data is newly formed in a frame buffer in addition to the normal data storage region for saving the normal pixel data. When an image is displayed, the compression data storage region is utilized so as to reduce a memory bandwidth required in pixel data reading operation.